The invention relates to a method for reducing aerosol emissions in a urea granulation plant with a recovery of the resulting scrubber bleeds. The method describes a known production of urea granulates in a granulator connected with a urea isomerisation unit which removes the ammonium isocyanate from the gas stream. The ammonium isocyanate in the bleed from the granulator is in a further step partly hydrolysed and partly isomerised to urea. The isomerised urea is given into a liquid phase which is then returned into the dust removing or scrubbing system. The invention also relates to a device for the production of urea granulates which makes a use of the related method.
Urea is usually produced by crystallizing a concentrated urea melt. The melt is introduced into a granulator which carries out a granulation at elevated temperature and evaporates the water in the melt. The resulting granulated particles are usually obtained in a shape which makes them ready for a use in the desired applications. Due to the high temperature, a portion of the urea is converted into ammonium isocyanate according to a reversible reaction. The respective chemical equation is:CO(NH2)2⇄NH4NCO  1.This reaction causes a part of the urea to decompose and produces ammonium isocyanate which enters the airflow leaving the granulator. This reaction is reversible with the consequence that the ammonium isocyanate which is the isomer of urea can be isomerised into the desired product urea. The ammonium isocyanate further decomposes with water to ammonium carbonate which at elevated temperature finally results in the formation of carbon dioxide, ammonia and water:NH4NCO+2H2O→(NH4)2CO3⇄2NH3+CO2+H2O  2.The final decomposition products carbon dioxide and ammonia, however, cannot be isomerised to urea but can be recycled into the urea production process.
To keep the overall process economic, it is a favourable option to separate the product urea from side products and to redirect them into the production process. The ammonium isocyanate is usually obtained as an aerosol so that separating the aerosol allows isomerising the ammonium isocyanate into urea. The gaseous side products carbon dioxide and ammonia are left over and can be redirected into the production process. In addition, the isocyanate is often emitted into the atmosphere which may be harmful to the environment. A process is therefore desired which also reduces or eliminates the emission of isocyanate into the environment.
The recovery of ammonia from off-gas of a urea-production is known. The WO 2005/032696 A1 describes a process for removing ammonia and dust from waste gas that is produced during the production of fertilizers, preferably urea. The waste gas is introduced into a first washer where it is washed with scrubbing water. To enable a distinct reduction of pollution caused by waste gas, both the waste gas from the granulator as well as the cooler gases from the granulator pass through at least one drop eliminator before exiting the washer. However, the invention gives no instruction on how to recycle the ammonium isocyanate that is a frequent byproduct of the granulation.
It is therefore desirable to find a process which captures the side products ammonium isocyanate, ammonia and water and which separates off the ammonium isocyanate from the side products from the urea granulation which is usually obtained as a separable aerosol or as fine particles. In addition, the desired process should isomerise the ammonium isocyanate into urea and redirect it into the production process. The desired process should also supply scrubbing systems for gases and supply a means for the recovery of the reaction heat.